This comes from Mark Sudduth, of hurricanetrack.com. Pretty neat little site, and he offers, free of charge, daily commentary:

As I look at conditions across the Atlantic Basin, it is clear that just about all of the ingredients needed to form a hurricane are in place. Water temps are plenty warm, upper level winds are favorable in several areas and there are tropical waves moving steadily off of Africa every few days. So what's the hold up? Dry, stable air. A layer of African dust and dry air is choking off any chance of deep convection developing right now in the deep tropics. Until that layer (called SAL or Saharan Air Layer) eases up, it is unlikely that we will see any development in the eastern Atlantic. However, over the next few weeks, as water temperatures get even warmer in the eastern Atlantic, the lower levels of the atmosphere will become more humid and unstable. It is no coincidence that the so-called "Cape Verde" season usually kicks off around the middle of August. So enjoy the calm right now- because there is still every indication that the peak of the season will be very busy.